Just get the injured unit near the medic (or vice versa), using the medic, move the cursor over the injured unit and you should see a heal icon appear. Click the action button and he will heal the unit (but not necessarily to full health, depending on the damage taken).

Get to know his weapons. Every Medic possesses a 40/150 Syringe Gun (Primary), the class defining Medi-Gun (Secondary) and their last resort, the Bonesaw (Melee). ...Use the Medi Gun to heal your teammates. ...Use your ĂśberCharge to make you and one of your teammates invincible for a short time.